1. Field of the Invention
The invention relates to field emission elements and manufacturing methods thereof and, particularly, to a field emission element employing carbon nanotubes and a manufacturing method thereof.
2. Description of Related Art
Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes are electrically conductive along their length, are chemically stable, and can each have a very small diameter (much less than 100 nanometers) and a large aspect ratio (length/diameter). Due to these and other properties, it has been suggested that carbon nanotubes can play an important role in fields such as microscopic electronics, field emission devices, thermal interface materials, etc.
Generally, a CNT field emission element includes a conductive cathode electrode and a carbon nanotube formed on the cathode electrode. The carbon nanotube acts as an emitter of the field emission element. The methods adopted for forming the carbon nanotube on the conductive cathode electrode mainly include mechanical methods and in-situ synthesis methods. One mechanical method is performed by using an atomic force microscope (AFM) to place a synthesized carbon nanotube on a conductive cathode electrode and to then fix the carbon nanotube on the conductive cathode electrode, via a conductive paste or adhesive. The mechanical method is relatively easy/straightforward. However, the precision and efficiency thereof are relatively low. Furthermore, the electrical connection between the conductive base and the carbon nanotube tends to be poor because of the limitations of the conductive adhesives/pastes used therebetween. Thus, the field emission characteristics of the carbon nanotube are generally unsatisfactory.
One in-situ synthesis method is performed by coating metal catalysts on a conductive cathode electrode and directly synthesizing a carbon nanotube on the conductive cathode electrode by means of chemical vapor deposition (CVD). The in-situ synthesis method is relatively easy. Furthermore, the electrical connection between the conductive base and the carbon nanotube is typically good because of the direct engagement therebetween. However, the mechanical bonding between the carbon nanotube and the conductive base often is relatively weak and thus unreliable. Thus, in use, such a carbon nanotube is apt, after a period of time, to break away (partially or even completely) from the conductive cathode electrode, due to the mechanical stress associated with the electric field force. Such breakage/fracture would damage the field emission electron source and/or decrease its performance. Furthermore, in the in-situ synthesis method, controlling of the growth direction of the carbon nanotube is difficult to achieve during the synthesis process. Thus, the production efficiency thereof can be relatively low, and the controllability thereof is often less than desired. Still furthermore, the in-situ synthesis method has a relatively high cost.
What is needed, therefore, is a field emission element that promotes a good mechanical and electrical connection between the carbon nanotube and the conductive cathode electrode and that, thus, tends to have satisfactory field emission characteristics.
What is also needed is a method for manufacturing the above-described field emission electron source, the method having a relatively low cost, relatively high production efficiency, and an improved controllability.